This invention relates to a reflectance control coating system. More particularly, this invention concerns itself with a coating system which exhibits high absorptance with transmittance and reflectance neaar zero for infrared energy.
A problem often encountered in the utilization of detector arrays involves the creation of undesirable optical crosstalk between the individual detectors within the array. Previous attempts at overcoming this problem involved the use of black paints in order to provide proper reflectance control. Unfortunately, the so called black paints are susceptible to outgassing or particle shedding in vacuum. Also, these coatings do not possess the high degree of absorption necessary to effectively control the undesirable optical crosstalk encountered when using detector arrays.
Another method suggested for controlling the crosstalk problem utilized a coating system design based on the use of silicon monoxide, chromium and opaque alminum. This system was optimized for absorption in the visible spectrum and reflection in the infrared spectrum. However, since this system exhibited a low infrared emittance and high visible absorbance, it did not function with the degree of efficiency needed to prevent optical crosstalk between the detectors. Furthermore, chromium coatings exhibit high stress and lack stability during time, humidity and thermal cycling processing procedures.
With the present invention, however, it has been found that a coating system composed of an opaque layer of titanium onto which is deposited a dielectric layer composed of a quarter wave optical thickness (QWOT) of alumina, followed by the deposition of a thin, semitransparent layer of titanium plus a final QWOT of alumina overcomes the problems of prior art systems and provides a coating system characterized by high absorption and near zero reflectance in the infrared range. The system is especially adaptable for application to refractory substrates, such as sapphire. The coating achieves its effective high absorption by trapping radiation within a resonant cavity constructed from the highly reflective opaque titanium layer and the semitransparent titanium layer, which also absorbs radiation at each bounce. A final anti-reflective layer fabricated from the same aluminum oxide material and same QWOT as the intermediate dielectric layer is deposited onto the semitransparent titanium layer.